When Paul declares the gospel in 1 Cor. 15:3-4 (the whole sentence is from v. 3 to v. 8), he is not saying that these three things: the death, burial, and resurrection of Jesus Christ, are all there is to the Gospel (though it is certainly not less than those three things). There have been false teachers throughout Christian history who have affirmed Jesus’ literal death, burial, and bodily resurrection.
Why then mention those three? Context gives us a clue. He is going to address their doubts about the truth (and thus the importance) of the believer’s resurrection and so he goes back to the nistorical elements that are foundational to the gospel.
The gospel cannot be separated from these historical events. The rest of the gospel is built upon these historical facts, explaining the meaning and significance of the events.